Huge Ancient Civilization's Collapse Explained

Below:

Next story in Science

The mysterious fall of the largest of the world's earliest urban
civilizations nearly 4,000 years ago in what is now India,
Pakistan, Nepal and Bangladesh now appears to have a key culprit
— ancient climate change, researchers say.

Ancient Egypt and Mesopotamia may be the best known of the first
great urban cultures, but the largest was the Indus or Harappan
civilization. This culture once extended over more than 386,000
square miles (1 million square kilometers) across the plains of
the Indus River from
the Arabian Sea to the Ganges, and at its peak may have
accounted for 10 percent of the world population. The
civilization developed about 5,200 years ago, and slowly
disintegrated between 3,900 and 3,000 years ago — populations
largely abandoned cities, migrating toward the east.

"Antiquity knew about Egypt and Mesopotamia, but the Indus
civilization, which was bigger than these two, was completely
forgotten until the 1920s," said researcher Liviu Giosan, a
geologist at Woods Hole Oceanographic Institution in
Massachusetts. "There are still many things we don't know about
them." [ Photos:
Life and Death of Ancient Urbanites ]

Nearly a century ago, researchers began discovering numerous
remains of Harappan settlements along the Indus River and its
tributaries, as well as in a vast desert region at the border of
India and Pakistan. Evidence was uncovered for sophisticated
cities, sea links with Mesopotamia, internal trade routes, arts
and crafts, and as-yet undeciphered writing.

"They had cities ordered into grids, with exquisite plumbing,
which was not encountered again until the Romans," Giosan told
LiveScience. "They seem to have been a more democratic society
than Mesopotamia and Egypt — no large structures were built for
important personalitiess like kings or pharaohs."

Like their contemporaries in Egypt and Mesopotamia,
the Harappans, who were named after one of their largest
cities, lived next to rivers.

"Until now, speculations abounded about the links between this
mysterious ancient culture and its life-giving mighty rivers,"
Giosan said.

Now Giosan and his colleagues have reconstructed the landscape of
the plain and rivers where this
long-forgotten civilization developed. Their findings
now shed light on the enigmatic fate of this culture.

The researchers first analyzed satellite data of the landscape
influenced by the Indus and neighboring rivers. From 2003 to
2008, the researchers then collected samples of sediment from the
coast of the Arabian Sea into the fertile irrigated valleys of
Punjab and the northern Thar Desert to determine the origins and
ages of those sediments and develop a timeline of landscape
changes.

"It was challenging working in the desert — temperatures were
over 110 degrees Fahrenheit all day long (43 degrees C)," Giosan
recalled.

After collecting data on geological history, "we could reexamine
what we know about settlements, what crops people were planting
and when, and how both agriculture and settlement patterns
changed," said researcher Dorian Fuller, an archaeologist with
University College London. "This brought new insights into the
process of eastward population shift, the change towards many
more small farming communities, and the decline of cities during
late Harappan times."

Some had suggested that the Harappan heartland received its
waters from a large glacier-fed Himalayan river, thought by some
to be the Sarasvati, a sacred river of
Hindu mythology. However, the researchers found that only
rivers fed by monsoon rains flowed through the region.

Previous studies suggest the Ghaggar, an intermittent river that
flows only during strong monsoons, may best approximate the
location of the Sarasvati. Archaeological evidence suggested the
river, which dissipates into the desert along the dried course of
Hakra valley, was home to intensive settlement during Harappan
times.

"We think we settled a long controversy about the mythic
Sarasvati River," Giosan said.

Initially, the monsoon-drenched rivers the researchers identified
were prone to devastating floods. Over time, monsoons weakened,
enabling agriculture and civilization to flourish along flood-fed
riverbanks for nearly 2,000 years.

"The insolation — the solar energy received by the Earth from the
sun — varies in cycles, which can impact monsoons," Giosan said.
"In the last 10,000 years, the Northern Hemisphere had the
highest insolation from 7,000 to 5,000 years ago, and since then
insolation there decreased. All climate on Earth is driven by the
sun, and so the monsoons were affected by the lower insolation,
decreasing in force. This meant less rain got into continental
regions affected by monsoons over time." [ 50
Amazing Facts About Earth ]

Eventually, these monsoon-based rivers held too little water and
dried, making them unfavorable for civilization.

"The Harappans were an enterprising people taking advantage of a
window of opportunity — a kind of "Goldilocks civilization,"
Giosan said.

Eventually, over the course of centuries, Harappans apparently
fled along an escape route to the east toward the Ganges basin,
where monsoon rains remained reliable.

"We can envision that this eastern shift involved a change to
more localized forms of economy — smaller communities supported
by local rain-fed farming and dwindling streams," Fuller said.
"This may have produced smaller surpluses, and would not have
supported large cities, but would have been reliable."

This change would have spelled disaster for the cities of the
Indus, which were built on the large surpluses seen during the
earlier, wetter era. The dispersal of the population to the east
would have meant there was no longer a concentrated workforce to
support urbanism.

"Cities collapsed, but smaller agricultural communities were
sustainable and flourished," Fuller said. "Many of the urban
arts, such as writing, faded away, but agriculture continued and
actually diversified."

These findings could help guide future archaeological
explorations of
the Indus civilization. Researchers can now better guess
which settlements might have been more significant, based on
their relationships with rivers, Giosan said.

It remains uncertain how monsoons will react to modern
climate change. "If we take the devastating floods that
caused the largest humanitarian disaster in Pakistan's history as
a sign of increased monsoon activity, than this doesn't bode well
for the region," Giosan said. "The region has the largest
irrigation scheme in the world, and all those dams and channels
would become obsolete in the face of the large floods an
increased monsoon would bring."

The scientists detailed their findings online May 28 in the
journal Proceedings of the National Academy of Sciences.